The invention relates to detecting faults in a resolver and in particular to a method and apparatus for detecting faults in a resolver based on the sum of the squared outputs of the resolver. Resolvers are position sensing devices having one excitation winding and two output windings, namely a sine winding and a cosine winding. Due to the physical relationship between the two output windings and the excitation winding, the voltage amplitude of the sine winding is proportional to the sine of the resolver angle and the voltage amplitude of the cosine winding is proportional to the cosine of the resolver angle. Because of the proportionality of the sine and cosine winding voltage amplitudes, the sum of the square of the sine voltage and the square of the cosine voltage should equal a constant.
Existing systems compare the sum of squared cosine and sine voltages to range limits in order to detect sensor faults. The high and low range limits were set wide enough to account for all worst case system variables. As a result, these limits would only detect extreme sensor faults (e.g., electrical opens or shorts). Small signal drifts (e.g., series resistance, resistance to ground, etc.), referred to as soft faults, could produce significant sensed position error but not be detected by the sum of squares range test.